The new findings in mice illustrate that the rodents can be made to fear a cage by giving it a foot shock while at the same time reactivating a memory of the cage to associate the two.

"The kinds of things that once existed only in the realm of science fiction movies like Inception and Eternal Sunshine of the Spotless Mind are now experimentally possible," said Steve Ramirez, a graduate student at Howard Hughes Medical Institute HHMI and first author.

Researchers knew that memories are stored by the brain in a small set of neurons. Understanding how this information is encoded could be key to understanding how human memory works as well as memory disorders.But identifying exactly which neurons are linked to specific memories has been technically challenging.

Scientists wondered if they could alter the way a mouse remembered a setting by activating neurons associated with it. They chose to test this idea by attempting to change whether or not a mouse was afraid of a particular cage.

To see whether they could make an animal associate fear with a previously neutral setting, Tonegawa's lab group first exposed mice to one of four unique cages. Each cage had distinct flooring materials, artificial smells, and different lighting. As the mice scouted out the new room, whichever neurons were activated produced the special light-sensitive protein.

The mice were moved to a second cage. This time, as the mice explored, the scientists used light to turn on the neurons that had been activated in the first cage and simultaneously shocked the feet of the mice. Then the mice were put back in the first area – where they'd never received a shock. The mice were clearly fearful of the setting, Ramirez said, spending more than a quarter of their time frozen in place.

"We were astonished that this worked on the very first mouse we ever tried. We got the animal to be scared of an environment where technically, nothing bad had ever happened to it," said Ramirez.

By contrast, when the mice were put in a third cage that they'd never been in before, they exhibited no fear. And in a control group of mice that had received shocks in the second cage but no neuron reactivation, the first cage never induced fear.